The present invention relates generally to particulate traps for exhaust treatment systems in diesel engines and, in particular, to an open end diesel particulate trap.
Exhaust treatment systems for diesel engines are well known. Exhaust treatment systems generally include a particulate filter in the exhaust piping that is utilized to remove particulates, which are typically composed of partially burned hydrocarbons, from the engine exhaust stream. The particulate filter includes a housing and a filter element disposed therein, known in the art as a ceramic wall-flow monolith particulate filter. A typical ceramic wall-flow monolith particulate filter includes an outer wall interconnected by a large number of interlaced, thin porous internal walls that define a honeycomb structure to provide parallel channels extending along the length of the outer wall from a one end of the filter to an opposite end of the filter. Exhaust gas enters the one end of the filter and exits through the opposite end. Alternate cell channel openings on the one end of the filter are blocked and, at the opposite end the alternate channel openings are blocked in a similar manner but displaced by one cell, which defines a plurality of parallel inlet cells and outlet cells. With this filter arrangement, the exhaust gas cannot flow directly through a given inlet cell because of the blocked ends and is forced to flow through the separating porous walls into an adjacent outlet cell. The exhaust gas is filtered as it flows through the porous walls between adjacent cells because the thin porous walls of the monolith trap the particulate. A typical particulate filter has a cell density ranging from 100 to 300 cells per square inch and has a length longer than 6 inches. These long, slim channels and closed ends allow this kind of filter to trap particulate very effectively, often trapping particulates in the size range of less than 0.1 micron.
Eventually, though, the cells of the filter become clogged with the trapped particulate and the filter must be cleaned in order to remain effective. The filter is typically cleaned utilizing a thermal generation process, such as by introducing a heat source to the cells to raise the temperature of the cells enough to cause the trapped particulate to oxidize. These traditional filters, however, are very difficult to clean thermally because of the long slim channels and because of the closed ends. In addition to the use of the thermal regeneration process, filters can be cleaned manually, which is also made difficult by the slim channels and closed ends. In addition, because of the blocked ends, airflow through the filter drops to zero adjacent the blocked end, which results in an uneven concentration of particulates with a heavier concentration of particulates located at the blocked ends. Ash, which is metallic components in the diesel particulate, can not be cleaned utilizing the thermal regeneration process and, therefore, must be cleaned manually, resulting in high maintenance costs.
It is desirable, therefore, to provide a particulate filter for diesel engines that is efficient and easy to clean either mechanically or thermally.
The present invention concerns a particulate filter operable to be installed in an exhaust system of an internal combustion engine, such as a diesel engine. The particulate filter according to the present invention includes a housing having an inlet and an outlet. The inlet of the housing is preferably connected to piping extending from the diesel engine. The outlet of the housing is preferably connected to piping that extends to atmosphere. The filter housing defines an exhaust gas path from the inlet to the outlet. A first portion of the exhaust gas path leads from the inlet and extends at an angle to a second portion of the exhaust gas path leading to the outlet. Preferably, the first portion of the exhaust gas path is approximately perpendicular to the second portion of the exhaust gas path. At least one elongated, generally tubular filter element is disposed in the housing and has a wall surrounding a central aperture extending between first and second ends. The wall of the filter element and the wall of the housing define an outlet chamber therebetween. The first end of the filter aperture is connected to the housing inlet and the first path portion extends along a longitudinal axis of the aperture. A particulate reservoir is attached to the housing and is connected to the second end of the filter element aperture. When the exhaust gas enters the inlet, flows along the aperture and through wall of the at least one filter element, enters the outlet chamber, and exits the outlet, some of the particulates in the exhaust gas remain in the at least one filter element and are burned when the at least one filter element is heated, creating ash which drops to and is collected in the particulate reservoir. Other particulates in the first path portion can fall into the particulate reservoir as the exhaust gas changes direction to follow the second path portion.
The filter is preferably of the ceramic filter type. In one embodiment, the filter is a single filter element that has the central aperture formed therein. Alternatively, the filter is a plurality of wall-flow type filters elements each including a central aperture and being positioned to provide parallel gas flow paths.
In operation, the particulate filter is installed in the exhaust system of the internal combustion engine. The inlet of the housing is connected to piping extending from the internal combustion engine and the outlet of the housing is connected to piping that extends to atmosphere. The engine is operated normally, which produces an exhaust stream that contains particulates entrained therein. The exhaust stream enters the inlet of the housing, travels along the central aperture along the first path portion, travels through the wall of the filter element along the second path portion, enters the outlet chamber, and exits the outlet of the housing.
During operation, some of the entrained particulate may drop from the exhaust gas travelling along the first path portion into the particulate reservoir due to gravitational force. The entrained particulate in the exhaust gas passing through the wall can be trapped by the filter element. Because there is no closed end, the exhaust flow velocity at the bottom end of the wall-flow filter is advantageously not zero. Consequently, the particulate will deposit more evenly along the axial length of the wall-flow filter element. If a thermal regeneration is initiated from the upstream side of the filter element, the hot oxidant can reach the bottom end of the filter easily because of the open end, thus producing a more thorough regeneration. After thermal regeneration, the metallic ash can drop into the particulate reservoir, again due to gravitational force. The normal vibration of the vehicle may also shake some of the deposited particulate and ash off from the filter surface. The particular geometry of the filter and the exhaust gas path allows the flaked-off particulate to drop down into the particulate reservoir. When the particulate reservoir is removed from the housing, the filter surface is accessible due to the open end, which allows the filter surface to be cleaned mechanically.
Another advantage of the particulate filter according to the present invention over prior art particulate filters is that more frontal open area can be assigned to the intake side of the filter than to the exhaust side of the filter. Prior art particulate filters typically have the same area for the intake and exhaust sides. Since particulate will deposit on the surface of the intake side, one needs more frontal open area to minimize the flow restriction.
The particulate filter according to the present invention is more durable and consumes less energy during thermal regeneration than prior art particulate filters. For applications that require mechanical regeneration and cleaning, the present invention provides the maximum advantage over the prior art particulate filters.